Blades made of steel or titanium materials are currently used for those components of thermal turbomachines—e.g. gas turbines, steam turbines and compressors—which are subject to centrifugal forces. In contrast, fan blades of aircraft engines—which are also included among thermal turbomachines—are produced completely or partially from fiber composite materials.
Although, as metal materials, steel and titanium are of high strength, they are also subject to high centrifugal forces owing to their relatively high density. Thus, the achievable flow cross sections are limited, and this limits the power rating of the machines in particular.
However, the use of fiber composite materials, which are of course lighter and therefore subject to lower centrifugal forces, is not suitable in all areas of application. Thus, in the case of steam turbines for example, there is the risk of “droplet impact”, i.e. the impingement of small water particles on the surface of the blade. Fiber composite materials would be destroyed within a short time by this action, and not only the fibers themselves but also the matrix structures connecting the fibers.
Another problem is that fiber composite materials are composed of aligned fiber plies laid one on top of the other. This means that each fiber ply is aligned substantially in a single direction of extent and therefore also ensures a particular stability particularly in relation to this selected direction of extent. If, on the other hand, fiber composite materials are supposed to ensure stability in several directions of extent, it is necessary to lay a plurality of fiber plies one on top of the other and to connect them, in which case the fiber plies must be aligned in different directions. In order to achieve a truly stable system, it is therefore often necessary to connect a large number of fiber plies to one another, and this may lead to excessive thicknesses.
It must therefore be stated that, on the one hand, metals (especially high-strength and corrosion-resistant metals such as steel or titanium) would be particularly suitable for constructing blades for turbomachines owing to their stability properties (resistance to centrifugal and torsional forces, tensile strength and yield strength and special hardness properties and resistance to the effects of foreign objects) and, on the other hand, fiber composite materials would be particularly advantageous for constructing blades for turbomachines because of their density properties. However, these two materials are virtually impossible to combine with one another in any useful way, for which reason there is the classic dilemma in the present case between two technical directions of approach which ultimately are almost mutually exclusive. Thus far, therefore, there is no fully satisfactory concept of how blades of thermal turbomachines can be produced that are, on the one hand, stable enough and, on the other hand, light enough.